Medial Collateral Ligament Tear

The medial collateral ligament (MCL) is injured when a valgus force  is applied to the flexed knee. There is often contusion of the lateral femoral condyle and/or tibial plateau as seen here.
Coronal spin density and STIR images are shown:

MCL tears are classified as follows:
Grade I - minimal tear without instability
Grade II - partial tear with increased instability
Grade III - complete rupture with gross instability

Complete rupture of the fibres is seen in the above case indicating a Grade III tear.

The following case shows a severe but not complete tear of the MCL since a few of the superficial most fibres remain intact (image C). This corresponds to a Grade II tear. The tear is seen as high signal (white) on the STIR sequence (image I). As in the above case there is contusion of the lateral tibial plateau. In this case there is an associated hairline fracture with cortical break (images J & K).

Images A to E coronal spin density, images F to I coronal STIR, J & K sagittal T2*: